This invention relates to power MOSFETs and more specifically relates to a power MOSFET having integrated on the same semiconductor die, novel over-temperature and overcurrent protection circuits.
Power MOSFETs are well known in the art. A particularly well-known power MOSFET is manufactured and sold by International Rectifier Corporation of El Segundo, Calif., under its registered trademark HEXFET. Such power MOSFETs are adapted to handle electrical power of at least about 1 watt and can be switched on and off by applying and removing a low voltage control signal between the gate and source electrodes of the device. They are packaged in various package styles, for example, a TO-220 style package which typically presents three terminals, gate, source and drain, for connection in an electrical circuit.
It is known to be desirable to turn off a power MOSFET, or otherwise control the circuit in which it is connected, when the current flow between the source and drain electrodes exceeds a particular value, for example, 5 times rated current, or when the die temperature exceeds a certain value, for example 150.degree. C. Some known devices act to turn off a power MOSFET in response to either high current or high temperature, and others respond to either condition.
For example, one known device employs a small bipolar thyristor die which is glued to the power MOSFET die and turns on to connect the MOSFET gate to the MOSFET source when the MOSFET die temperature exceeds a given value. This device, however, has too long a thermal time constant to offer short circuit protection.
Another known device employs a small bipolar transistor chip with emitter and collector electrodes connected between the power MOSFET gate and source terminals. The bipolar transistor base to emitter circuit is connected across a voltage dropping resistor in the power MOSFET source circuit, so that the bipolar transistor turns on under short circuit conditions to short the power MOSFET gate to the source and limit current. However, if the power FET is exposed to a high current which is less than short circuit current, the control bipolar transistor simply reduces the power MOSFET drive, causing very high power dissipation and possible destruction of the device. Moreover, this circuit does not monitor the temperature of the power MOSFET die.
A third known device employs a current-sensing power MOSFET which has a limited number of cells of the MOSFET devoted to production of an output signal which is representative of the main power current, and also has a temperature monitoring circuit to monitor the temperature of the MOSFET die. The circuits for these functions are integrated into the power MOSFET die, but require an auxiliary power source to power these circuits. This auxiliary power is applied to the power MOSFET via a fourth terminal, in addition to the source, drain and gate terminals. Consequently, the device is not pin compatible with existing power MOSFETs.
A fourth known device provides current and temperature sensing and has only three pins (gate, source and drain) but this device employs a 4K voltage dropping resistor connected between the gate pin and gate electrode in order to limit the input current in case of a fault. The protect circuit then operates to turn on an auxiliary MOSFET connected between the gate to source terminals of the power MOSFET to turn off the power MOSFET under given overcurrent or over-temperature conditions.
The voltage dropping resistor in the above 10 device is subject to stringent design tradeoffs. Thus, if the resistor value is too low, a high pin current will be drawn through the turn-off MOSFET. If the resistor value is too high, switching time is increased for the main power FET.